Zener diodes are some of the most extensively-used components in semiconductor technology, being used for a wide variety of applications, including voltage regulation and protection from electrostatic discharge events. Two different kinds of breakdown current may affect the operation of a diode at breakdown: impact ionization, or avalanche, breakdown current, and tunneling, or zener, breakdown current. The term “zener diode,” as it is classically used, and as it will be used herein, refers to a diode that is being operated in reverse bias where both tunneling breakdown and avalanche breakdown occur simultaneously. Given this classical definition, it is improper to use the term “zener diode” to refer to a diode with a breakdown voltage above approximately six volts, because such a diode does not experience tunneling effects, as will be further explained below. Impact ionization breakdown current has a positive temperature coefficient, meaning the breakdown voltage increases with temperature. Tunneling current has a negative temperature coefficient, meaning the breakdown voltage decreases with temperature. Diodes having breakdown voltages below approximately five volts are dominated by tunneling breakdown current, while diodes having breakdown voltages above approximately six volts are dominated by impact ionization breakdown current. Diodes having breakdown voltages between approximately five and six volts simultaneously experience both tunneling breakdown current and impact ionization breakdown current, and the temperature coefficients of the impact ionization breakdown current and the tunneling breakdown current tend to cancel each other out. This cancellation effect produces a diode with a breakdown voltage that is relatively insensitive to temperature.
In power integrated circuit (IC) technology, conventional manufacturing techniques typically require that zener diode junctions be placed under field oxide regions. When a diode is biased into breakdown, the many energetic (hot) carriers that are generated may be injected into the overlying oxide region, thereby modifying the electric field profile of the diode junction, which in turn changes the breakdown voltage of the diode. In this way, the voltage clamp of a zener diode can move, or drift, over time. In some applications, such as gate to source voltage clamping, voltage clamping for electrostatic discharge (ESD) protection, and clamped inductive switching of power devices, such drifting of the zener voltage clamp can be problematic. Some of the above-mentioned applications require a voltage clamp that is constant to within approximately 10 milliamps.
A zener diode formed by adjacent, heavily-doped (approximately 1×1020 atoms per centimeter cubed) shallow semiconductor regions typically constitutes a very leaky diode that, according to the classical definition given above, is not even a true zener diode because it has a breakdown voltage of approximately three to four volts. The implantation steps used to form the P and N regions of such a diode mechanically damage the silicon at the zener diode junction between the P and N regions to the point where the performance of the diode is compromised. The increasing use of silicidation in power IC technology further complicates the formation of zener diodes because the silicide tends to short the zener diode junction. Furthermore, if zener diodes are to be stacked in series for voltage clamping purposes, the zener diodes must be isolated. Such isolation can consume a lot of space. Accordingly, there is a need for a self-isolating zener diode with a junction that may be formed in an active area and that is compatible with silicide technology.
For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of embodiments of the present invention. The same reference numerals in different figures denote the same elements.
The terms “first,” “second,” “third,” “fourth,” and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms “comprise,” “include,” “have,” and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,” “under,” in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. The term “coupled,” as used herein, is defined as directly or indirectly connected in an electrical or non-electrical manner.